Digital systems use clocks to drive or step operation of sequential and latching digital circuits, and information transfer over data buses of the digital system. The speed at which the digital circuits reliably operate is dependent upon the supply voltage to these digital circuits. Higher operating voltages allow faster clock speeds for operation of the digital circuits. However, there are digital systems that may be battery operated and must continue operation even when the battery voltage degrades. Also faster clock speeds cause the digital circuits to draw more power.
Universal serial bus (USB) devices are being used for many applications in digital systems. The universal compatibility of USB devices for many different types of digital devices that may be used in a variety of applications. However, these diverse applications impose a broad range of operating parameter capabilities required by the USB devices, e.g., computer operation at full voltage and power versus remote field operation using a battery voltage supply. The former benefits from high data throughput (fast clock speeds) and the latter benefits from low operating voltage and power consumption (slow clock speeds).
Therefore, there is a need for digital devices having USB interfaces that will operate over a wide range of supply voltages and power consumption.